1. Field of the Invention
The present invention relates in general to digital communication systems and in particular to mobile radio systems. Still more particularly, the invention relates to a method of reception and a receiver in a mobile radio system operating in a single antenna interference cancellation environment.
2. Description of the Related Art
The most widespread standard in cellular wireless communications is currently the Global System for Mobile Communications (GSM). GSM employs a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) for the purpose of sharing the spectrum resource. GSM networks typically operate in the 900 MHz frequency range. Radio spectrum in the 890-915 MHz bands is for the uplink (mobile station to base station) and in the 935-960 MHz bands is for the downlink (base station to mobile station). The spectrum for both uplink and downlink is divided into 200 kHz-wide carrier frequencies using FDMA, and each base station is assigned one or more carrier frequencies. Each carrier frequency is divided into eight time slots using TDMA. Eight consecutive time slots form one TDMA frame, with a duration of 4.615 ms. A physical channel occupies one time slot within a TDMA frame. Each time slot within a frame is also referred to as a burst. TDMA frames of a particular carrier frequency are numbered, and formed in groups of 26 or 51 TDMA frames called multi-frames.
GSM systems typically employ one or more modulation schemes to communicate information such as voice, data, and/or control information. These modulation schemes may include GMSK (Gaussian Minimum Shift Keying), M-ary QAM (Quadrature Amplitude Modulation) or M-ary PSK (Phase Shift Keying), where M=2n, with n being the number of bits encoded within a symbol period for a specified modulation scheme. GMSK is a constant envelope binary modulation scheme allowing raw transmission at a maximum rate of 270.83 kilobits per second (kbps). While GSM is sufficient for standard voice services, high-fidelity audio and data services demand higher data throughput rates.
General Packet Radio Service (GPRS) is a non-voice service that allows information to be sent and received across a mobile telephone network. It supplements Circuit Switched Data (CSD) and Short Message Service (SMS). GPRS employs the same modulation schemes as GSM, but higher data throughput rates are achievable with GPRS since it allows for all eight time slots to be used by a single mobile station at the same time.
The EDGE (Enhanced Data rates for GSM Evolution) and the associated packet service EGPRS (Enhanced General Packet Radio Service) have been defined as a transitional standard between the GSM/GPRS (Global System for Mobile Communications/General Packet Radio Service) and UMTS (Universal Mobile Telecommunications System) mobile radio standards. Both GMSK modulation and 8-PSK modulation are used in the EDGE standard, and the modulation type can be changed from burst to burst. GMSK is a non-linear, Gaussian-pulse-shaped frequency modulation, and 8-PSK modulation in EDGE is a linear, 8-level phase modulation with 3π/8 rotation. However, the specific GMSK modulation used in GSM can be approximated with a linear modulation (i.e., 2-level phase modulation with a π/2 rotation). The symbol pulse of the approximated GSMK and the symbol pulse of the 8-PSK are identical.
Wireless communication systems have an ever-increasing demand on capacity to transfer both voice and data services. In GSM communication systems, one way to increase system capacity is to increase the frequency reuse factor, whereby the communications system allocates the same frequency to multiple sites in closer proximity. However, stray signals or signals intentionally introduced by frequency reuse methods can interfere with the proper transmission and reception of voice and data signals and can lower capacity. As a result, a receiver must be capable of processing a signal with interference from other channels and extracting the desired information sent to a user.
It is well known that the major source of noise and interference experienced by GSM communication devices operating in typical cellular system layouts supporting a non-trivial number of users is due to co-channel or adjacent channel interference. Such noise sources arise from nearby devices transmitting on or near the same channel as the desired signal or from adjacent channel interference such as noise arising on the desired channel due to spectral leakage, for example. Additionally, even in the case where no other signal interference is present, the received signal may consist of multiple copies of the transmitted data sequence due to multi-path channel conditions, for example. This effect is sometimes referred to as self-interference.
Traditionally, interference cancellation techniques have had limited success focusing on adjacent channel suppression by using several filtering operations to suppress the frequencies of the received signal that are not also occupied by the desired signal. Correspondingly, co-channel interference techniques have been proposed, such as joint demodulation, which generally require joint channel estimation methods to provide a joint determination of the desired and co-channel interfering signal channel impulse responses. Given known training sequences, all the co-channel interference can be estimated jointly. However, this joint demodulation requires a large amount of processing power, which constrains the number of equalization parameters that can be used efficiently.
A recently proposed standard for advanced communications systems and receiver algorithms called Single Antenna Interference Cancellation (SAIC) is designed for the purpose of improving system capacity through increasing frequency reuse. SAIC performs in the presence of co-channel interference resulting from the increased frequency reuse by enhancing single-antenna receiver performance. Current SAIC receiver algorithms are generally optimized for GMSK modulated signals, since gains of SAIC tend to be smaller for 8-PSK modulated signals. In an SAIC operational environment, GMSK traffic on neighboring cells can reuse common frequencies, thereby significantly increasing network bandwidth, while still tolerating the significantly higher co-channel and multi-channel interference than can be accommodated by conventional GMSK/EDGE environments.
In Additive White Gaussian Noise (AWGN) dominated environments, such as a conventional GSM/EDGE environment, Maximum Likelihood Sequence Estimation (MLSE) is the optimal solution. However, in co-channel and adjacent channel dominated environments (low carrier/interference (C/I) environments), such as a SAIC operational environment, a linear equalizer technique that takes advantage of the GMSK signal structure provides the superior performance. In actual SAIC operational environments, however, co-channel and adjacent channel interference is not always strong, depending on the traffic and system allocation. For example, to provide different quality of services, the system may allocate different data rates and coding schemes or allocate higher priority channel access by controlling the levels of co-channel and adjacent channel interference from other users. In other words, a particular SAIC enabled mobile station will be expected to operate in a very wide range of co-channel and adjacent channel interference levels. What is needed is an equalization methodology that provides superior performance in both high and low interference levels for mobile stations capable of operating in SAIC operational environments.